Make failure block optional

[junlee-qz]

Some how

- (NSURLSessionDataTask *)requestUrlWithRetryRemaining:(NSInteger)retryRemaining maxRetry:(NSInteger)maxRetry retryInterval:(NSTimeInterval)retryInterval progressive:(bool)progressive fatalStatusCodes:(NSArray<NSNumber *> *)fatalStatusCodes originalRequestCreator:(NSURLSessionDataTask *(^)(void (^)(NSURLSessionDataTask *, NSError *)))taskCreator originalFailure:(void(^)(NSURLSessionDataTask *task, NSError *))failure {

method doesn't specify originalFailure block as nullable which can cause crashes if we pass null

Original methods from AFNetworking take nullable blocks.
ex) https://github.com/AFNetworking/AFNetworking/blob/master/AFNetworking/AFHTTPSessionManager.m#L125